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This application relates to the continuous production of diaryl carbonates by reaction of dialkyl carbonates and an aromatic alcohol in the presence of a catalyst.
Diaryl carbonates, such as diphenyl carbonate, are an important reactant in the production of polycarbonate resins. As the uses to which polycarbonates are put have increased, the efficient production of diaryl carbonates has become of greater significance. Early processes for the production of diaryl carbonates utilized phosgene as a reagent. The toxicity of phosgene, however, prompted the development of a non-phosgene process. As shown schematically in FIG. 1, the non-phosgene process involves a two-step process. First, a dialkyl carbonate such as dimethyl carbonate (DMC) reacts with an aromatic alcohol such as phenol to produce an alkyl aryl carbonate (e.g., phenylmethyl carbonate) and an alkyl alcohol (methanol). Then, in the second step, two molecules of the alkyl aryl carbonate undergo a transesterification reaction to produce one molecule of diaryl carbonate (diphenyl carbonate, DPC) and one molecule of dialkyl carbonate (DMC).
Various methods and apparatus for making diaryl carbonates are known in the art. For example, U.S. Pat. No. 5,210,268, which is incorporated herein by reference, relates to a process for continuously producing aromatic carbonates. The process is carried out in a distillation column, with product being recovered from the bottom of the column, and low boiling by-products being removed via the top of the column. Other processes for production of diaryl carbonates using a series of distillation columns are disclosed in U.S. Pat. Nos. 5,344,954 and 5,705,673 which are incorporated herein by reference.
The reaction shown in FIG. 1 is the reaction which is desired, but as is known to persons skilled in the art, there are number of side reactions which occur, producing unwanted by-products. These by-products can interfere with continuing production of the desired product, reduce the efficiency of the over-all process, and in some cases produce waste streams which require special handling for disposal. Thus, a significant challenge to the utilization of this process is the development of a process which minimizes the quantities and effects of the reaction by-products, while providing a good yield of the desired product.
The present invention provides a method for continuous production of diphenyl carbonate which has a high production rate while at the same time maintaining an energy efficient process. The present invention further provides an apparatus for continuous production of diphenyl carbonate which has a high production rate while at the same time maintaining an energy efficient process.
The invention is directed to a method and apparatus for continuous production of diaryl carbonates by reaction of a dialkyl carbonate and an aromatic alcohol in the presence of a transesterification catalyst. The method comprises the steps of:
(a) introducing reactant streams containing dialkyl carbonate, aromatic alcohol and transesterification catalyst to a first reactive distillation column to produce alkyl aryl carbonate and alkyl alcohol;
(b) recovering from the first reactive distillation column a first top stream containing dialkyl carbonate and alkyl alcohol and a first bottom stream containing alkyl aryl carbonate;
(c) introducing the first bottom stream into a second reactive distillation column to produce diaryl carbonate by disproportionation of the alkyl aryl carbonate;
(d) recovering from the second reactive distillation column a first side stream containing dialkyl carbonate and alkyl aryl ether and a second bottom stream containing diaryl carbonate, alkyl aryl carbonate and dialkyl carbonate;
(e) introducing the first side stream into a second rectification column to separate a dialkyl carbonate stream from the alkyl aryl ether, and recycling the dialkyl carbonate stream to the first rectification column;
(f) introducing the second bottom stream to a third reactive distillation column to further drive the reaction toward diaryl carbonate;
(g) recovering from the third reactive distillation column a second top stream containing unreacted aromatic alcohol, dialkyl carbonate and alkyl aryl ether and recycling the second top stream to the first reactive distillation column;
(h) introducing the first top stream into a first rectification column;
(i) recovering from the first rectification column an azeotrope top stream consisting essentially of dialkyl carbonate/alkyl alcohol azeotrope and a third bottom stream containing dialkyl carbonate, and recycling the third bottom stream to the first reactive distillation column; and
(j) recovering a product stream containing essentially all of the diaryl carbonate produced from the bottom of the third reactive distillation column.